A common variety of locknuts are those which use a deformed (ovalized) nut which requires significant torque to thread the nut onto a bolt or stud. Deformed nuts wear the stud threads which can limit their useful life.
Another common variety of locknuts are those which use partial or full elastomer inserts which require significant torque to thread the nut onto a bolt or stud. However, use of elastomer inserts are very limited in high temperature applications and they mask faults in the bolt or stud being secured.
Some locking features are effective only as long as the stud threads are not worn. Once the stud threads are worn, the deformed locknut will lose some of its resistance torque.
Some locking features are effective only so long as the nut does not turn with respect to the bolt or stud. Once the nut turns some minimal amount, either due to temperature variations, shock or vibration, the nut will no longer be locked.
When a nut is tightened on the threads of a bolt or stud, the more torque applied, the more live load or preload that is passed through the threads of the bolt to create a tensile force in the bolt. To avoid disruption of the locking feature as a result of minimal rotation, many locknuts rely on a torque provided by a resilient preload, such as Belleville washers and the like.
During repair or overhaul procedures, the locknuts need be loosened, and thereafter reapplied to the same or similar bolts. In either case, the threads of either the locknut or the bolt may be worn as a consequence of the tensile stress applying frictional forces to the threads of the bolt and of the locknut. This wear can increase the likelihood of the locknut loosening by virtue of vibration, or in some instances, by virtue of rotary shock imparting torque to the locknut inadvertently. In some cases, nuts are secured to bolts after being fully tightened thereon by pinching, by deformation of the bolt and nut, or by damaging small areas of the interface between the bolt threads and with the nut threads, such as by denting with a punch or chisel. In the latter cases, the upper portion of the bolt receives as much wear as the nut whenever repair or other refurbishment occurs. Utilizing new locknuts does not necessarily overcome the likelihood of locknuts loosening thereafter. In cases where the bolt is in fact a stud, stud replenishment would be required in order to be fully effective.